A. Technical Field
The present invention relates to a water-absorbent resin powder and a production process therefor. More specifically, the present invention relates to a production process for a water-absorbent resin powder which is excellent in particle diameter distribution and properties, wherein the production process not only inhibits adhesion and aggregation in the production steps, but also is excellent in energy efficiency, drying efficiency, and productivity.
Furthermore, the present invention relates to a process for obtaining a water-absorbent resin powder as modified by adding an aqueous liquid to the resultant powder, which process is to more improve properties of the water-absorbent resin powder by bettering the uniform mixability of the aqueous liquid even if a special mixer or organic solvent is not especially used.
B. Background Art
In recent years, water-absorbent resins are widely used as components of sanitary materials, such as disposable diapers, sanitary napkins and incontinent pads, for the purpose of causing the water-absorbent resins to absorb much water. In addition, the water-absorbent resins are widely used not only for the sanitary materials, but also for the purpose of absorbing or retaining water, like water-retaining agents for soil and drip sheets for foods.
As to the above water-absorbent resins, the following are known as their examples: partially-neutralized and crosslinked poly(acrylic acids); hydrolyzed copolymers of starch-acrylonitrile; neutralized graft polymers of starch-acrylic acid; saponified copolymers of vinyl acetate-acrylic acid ester; hydrolyzed copolymers of acrylonitrile or acrylamide, or crosslinked polymers of these hydrolyzed copolymers; crosslinked polymers of carboxymethyl cellulose; crosslinked copolymers of 2-acrylamido-2-methylpropanesulfonic acid (AMPS); crosslinked poly(ethylene oxide); crosslinked polyallylamine; and crosslinked polyethylenimine. Many of them are used as powders.
The mainstream of production processes for these water-absorbent resins is a process comprising the steps of: polymerizing an aqueous monomer solution (containing a crosslinking agent, if necessary); and drying the resultant crosslinked hydrogel polymer; and further, if necessary, pulverizing the resultant dry polymer; thus obtaining a water-absorbent resin as a powder. However, because of high absorbency, tackiness, and adhesion, and inferior heat resistance, the crosslinked hydrogel polymer of the water-absorbent resin has disadvantages in that: drying and pulverizing thereafter are very difficult, the productivity is low, and the properties of the resultant water-absorbent resin and the energy efficiency are very bad.
Known examples of methods for drying such a crosslinked hydrogel polymer of the water-absorbent resin include: drying with a drum dryer (JP-A-053165/1979); a method in which a dry powdery acrylic acid polymer and a hydrogel are mixed together and then dried while being stirred (JP-A-117551/1982); azeotropic dehydration (JP-A-198714/1982); drying at a specific dew point (JP-A-026604/1989/U.S. Pat. No. 4,920,202); freeze-drying (JP-A-304127/1989, JP-A-304128/1989); a method which involves stir-drying in a cylindrical dryer (JP-A-240112/1990/U.S. Pat. No. 5,005,771); a method in which a gel is extruded through specific apertures and then dried (U.S. Pat. No. 5,275,773); microwave drying (JP-A-209010/1993/U.S. Pat. No. 5,075,344); a method which involves the use of a specific gel cutter followed by hot-wind drying (JP-A-230124/1993); a method which involves hot-wind drying while measuring the differential pressure (JP-A-073518/1996); a method which involves stir-drying after adding a surfactant (JP-A-134134/1996); and a method which involves static drying, and then pulverizing, and then stirring or fluidize-drying (JP-A-240914/1999/EP 0926162).
In addition, a drying method in which the crosslinked hydrogel polymer is dried in a state laminated on a punching metal or a metal gauze is known. However, after being dried, the resultant dry polymer displays bad releasability to adhere to or clog the metal gauze or the apertures, therefore particularly the hot-wind drying etc. have problems of much deterioration of the drying efficiency involved by deterioration of permeability. Thus, a method is also known in which method a special conveyer having pins is used for the purpose of inhibiting such adhesion and clogging (JP-A-270070/1995/DE 19511769).
Furthermore, besides properties (such as water absorption capacity, water-extractable content, and liquid permeability), the particle diameter distribution is important for the water-absorbent resin, and the importance of water-absorbent resins having specific narrow particle diameter distributions is also known (JP-A-132802/1989/U.S. Pat. No. 5,061,259, JP-A-196802/1990/U.S. Pat. No. 5,244,735, JP-A-191604/1990/U.S. Pat. No. 4,973,632, JP-A-507564/1994/U.S. Pat. No. 5,419,956, and EP 0629411). In addition, a water-absorbent resin having a plurality of particle diameter distributions is also known (EP 0845272 and JP-A-130978/1999).
Thus, efficient classification is demanded as a classification method for such an aimed particle diameter adjustment, and the following methods are also known as methods for classifying water-absorbent resins after drying them: a method which involves the use of a heated or heat-retained sieve (JP-A-202187/1998/EP 0855232); and a method which involves the use of a classifying gauze as coated with such as Teflon (JP-A-156299/1999). Also known is a method in which an undried product from the drying step is classified before or during the pulverization step (JP-A-292919/1989/EP 0948997).
However, even in these methods, because of high absorbency and tackiness and inferior heat resistance, the water-absorbent resin and its crosslinked hydrogel polymer have disadvantages in that: drying and pulverizing thereafter are very difficult, and the deterioration of properties and particle diameter distribution of the resultant water-absorbent resin are seen, and further, the energy efficiency and the productivity are very bad. In addition, there are problems in that aggregation of powder in the production steps is seen also after the pulverization or classification step, so that the production efficiency and the quality are deteriorated.
Furthermore, there is a known method in which the particle diameter distribution of the water-absorbent resin powder or its absorption properties under a load are improved by adding water only or an aqueous liquid containing an additive to the resultant polymer powder. Such a method is often used for such as granulation (U.S. Pat. No. 5,369,148), surface-crosslinking (U.S. Pat. No. 5,409,771, U.S. Pat. No. 5,422,405, and U.S. Pat. No. 5,597,873), and decrease of residual epoxy compounds (U.S. Pat. No. 5,981,070).
However, the water-absorbent resin absorbs water in a moment and thereby displays tackiness. Therefore, hitherto, the improvement of properties of the water-absorbent resin is insufficient because of nonuniform mixing of the aqueous liquid, and further, according to circumstance, continuous operation itself might be difficult because the water-absorbent resin, as aggregated due to the nonuniform mixing, adheres to the mixer. In addition, a polyhydric alcohol is favorable as the crosslinking agent or a solvent therefor also in respect to properties and safety, but it is difficult to uniformly mix the water-absorbent resin with aqueous liquids, particularly, an aqueous polyhydric alcohol solution because of its high hydrophilicity and viscosity.
Thus, there are known arts in which, when modifying a water-absorbent resin powder by adding thereto an aqueous liquid, a special mixer is used (EP 0450923, EP 0812873, etc.), or an inorganic powder is used (U.S. Pat. No. 4,587,308), or an organic solvent is also used by adding it into an aqueous liquid (U.S. Pat. No. 4,734,478). However, the use of organic solvents, particularly, volatile ones, involves problems of causing the deterioration of properties in addition to problems of cost, environment, and safety. Furthermore, there is a known art in which, when adding an aqueous liquid to a water-absorbent resin, properties (AUL/Absorbency under Load) of the resin are controlled in a specific range (WO 98/49221). However, in such a method, the water-absorbent resin powder to which this method is applied is also very much limited, and its production is also difficult.
A. Objects of the Invention
The present invention has been made in consideration of the above-mentioned prior art problems and, from the above-mentioned present circumstances, an object of the present invention is to provide a production process which can produce a water-absorbent resin powder having a narrow particle diameter distribution and high properties by efficiently drying or pulverizing a crosslinked hydrogel polymer and making a particle diameter adjustment without thermal deterioration, and which inhibits adhesion and aggregation of the water-absorbent resin powder in the production steps, and displays good energy efficiency and high productivity.
Furthermore, another object of the present invention is to more improve properties of the water-absorbent resin powder by adding a uniform aqueous liquid (particular, aqueous crosslinking agent solution) by more improving the mixability even if a special mixer or organic solvent is not used.
B. Disclosure of the Invention
The present inventors diligently studied to achieve the above-mentioned objects. As a result, they have completed the present invention by finding that a production process for a water-absorbent resin powder provides the following good results that: the energy efficiency is good; the aggregation of the powder does not occur in the production steps; the thermal deterioration and the adhesion are very little in the drying step; and the pulverization step following the drying step is also extremely efficiently done; wherein the production process comprises the steps of: polymerizing an aqueous monomer solution containing a crosslinking agent; heat-drying the resultant crosslinked hydrogel polymer; and pulverizing the resultant dry polymer; wherein:
the dry polymer resultant from the heat-drying step is cooled before or during the pulverization step; and
the water-absorbent resin powder has an average particle diameter of 200 to 600 xcexcm after the pulverization step wherein the total ratio of particles having particle diameters of not larger than 150 xcexcm and particles having particle diameters of not smaller than 850 xcexcm in the water-absorbent resin powder is not more than 15 weight % of the water-absorbent resin powder.
In addition, the present inventors have completed the present invention by further finding that if the water-absorbent resin powder is cooled and further if its bulk density is adjusted, then its mixability with aqueous liquids (particularly, aqueous crosslinking agent solution) can be improved to thereby improve the continuous operability and the properties.
That is to say, a production process for a water-absorbent resin powder, according to the present invention, comprises the steps of: polymerizing an aqueous monomer solution containing a crosslinking agent; heat-drying the resultant crosslinked hydrogel polymer; and pulverizing the resultant dry polymer; wherein:
the dry polymer resultant from the heat-drying step is cooled before or during the pulverization step; and
the water-absorbent resin powder has an average particle diameter of 200 to 600 xcexcm after the pulverization step wherein the total ratio of particles having particle diameters of not larger than 150 xcexcm and particles having particle diameters of not smaller than 850 xcexcm in the water-absorbent resin powder is not more than 15 weight % of the water-absorbent resin powder.
In addition, another production process for a water-absorbent resin powder, according to the present invention, comprises the steps of: polymerizing an aqueous monomer solution containing a crosslinking agent; heat-drying the resultant crosslinked hydrogel polymer; and pulverizing the resultant dry polymer;
with the production process further comprising the steps of: forcibly cooling the dry polymer after the heat-drying step; and recycling the resultant heat in order to utilize it for forcible heating in a production process for a water-absorbent resin.
In addition, another production process for a water-absorbent resin powder, according to the present invention, comprises the steps of: polymerizing an aqueous monomer solution containing a crosslinking agent; heat-drying the resultant crosslinked hydrogel polymer; pulverizing the resultant dry polymer; and adding an aqueous liquid to the resultant powder in a mixer; wherein:
the temperature of the heat-drying step is in the range of 110 to 230xc2x0 C.; the dry polymer is cooled into the range of 80 to 35xc2x0 C. before the addition of the aqueous liquid; and the dry polymer is adjusted so as to have a bulk density of not less than 0.65 g/ml after the pulverization step.
In addition, another production process for a water-absorbent resin powder, according to the present invention, comprises the steps of: polymerizing an aqueous monomer solution containing a crosslinking agent; heat-drying the resultant crosslinked hydrogel polymer; pulverizing the resultant dry polymer; and adding an aqueous liquid to the resultant powder in a mixer; wherein:
the temperature of the heat-drying step is in the range of 110 to 230xc2x0 C.; the dry polymer is cooled into the range of 80 to 35xc2x0 C. before the addition of the aqueous liquid; the mixer is a stirring mixer having an internal wall temperature of not lower than 40xc2x0 C.; and the mixture resultant from the addition of the aqueous liquid is reheated into the range of 110 to 230xc2x0 C.
In addition, another production process for a water-absorbent resin powder, according to the present invention, comprises the steps of: polymerizing an aqueous monomer solution containing a crosslinking agent; heat-drying the resultant crosslinked hydrogel polymer; and pulverizing the resultant dry polymer; wherein:
the dry polymer is forcibly cooled after the heat-drying step;
the step of pulverizing the dry polymer is carried out so as to form a water-absorbent resin powder having a bulk density of not less than 0.65 g/ml; and
the cooled water-absorbent resin powder is further surface-crosslinked.
In addition, another production process for a water-absorbent resin powder, according to the present invention, comprises the steps of: polymerizing an aqueous monomer solution containing a crosslinking agent; heat-drying the resultant crosslinked hydrogel polymer; and pulverizing the resultant dry polymer; wherein:
the dry polymer is forcibly cooled after the heat-drying step;
the step of pulverizing the dry polymer is carried out so as to form a water-absorbent resin powder having a bulk density of not less than 0.65 g/ml; and
an aqueous liquid is further added to the cooled water-absorbent resin powder in a stirring mixer having a heated internal wall.
In addition, a water-absorbent resin powder, according to the present invention, is obtained by any one of the above production processes according to the present invention, and exhibits an absorption capacity of not less than 25 g/g under a load (1.96 kPa). An absorbent article, according to the present invention, comprises this water-absorbent resin powder.
In addition, another water-absorbent resin powder, according to the present invention, is obtained by a process including the steps of: polymerizing an aqueous monomer solution containing a crosslinking agent; heat-drying the resultant crosslinked hydrogel polymer; pulverizing the resultant dry polymer; and adding an aqueous polyhydric alcohol solution to the resultant powder in a mixer wherein the aqueous polyhydric alcohol solution does not contain any volatile organic solvents; wherein the resultant water-absorbent resin powder has the following properties: (1) the bulk density is not less than 0.65 g/ml; (2) the average particle diameter is in the range of 200 to 600 xcexcm; (3) the total ratio of particles having particle diameters of not larger than 150 xcexcm and particles having particle diameters of not smaller than 850 xcexcm in the water-absorbent resin powder is not more than 10 weight % of the water absorbent resin powder; and (4) the absorption capacity under a load (1.96 kPa) is not less than 25 g/g. Another absorbent article, according to the present invention, comprises this water-absorbent resin powder.
These and other objects and the advantages of the present invention will be more fully apparent from the following detailed disclosure.